Heretofore, as a system for detecting that a worker comes excessively close to an apparatus capable of self-propelling, such as a forklift or a mobile construction machine (such an apparatus will hereinafter be referred to as “self-propelled apparatus”), there has been known a system using an ultrasonic wave and a radio wave, as disclosed in the following Parent Document 1, a system using an infrared (IR) sensor and a distance sensor, as disclosed in the following Parent Document 2, and a system using two radio waves, as disclosed in the following Parent Document 3. Further, the following Patent Document 4 discloses a system using an electromagnetic induction wave and a radio wave.